1. Field of the Invention
This invention relates to a lens holder for supporting an objective lens loaded on an optical head used for recording and/or reproducing an information recording medium, such as an optical disc. This invention also relates to a method for manufacturing such a lens holder, a metal die used in manufacturing a lens holder and an objective lens device constituted by the objective lens and the lens holder.
2. Description of the Related Art
The information recording mediums, such as a replay-only optical disc, a phase change disc, a magneto-optical disc or an optical card, are finding extensive use for storing the image information, speech information or data for computer programs. Thus, the demand for raising the recording density and the recording capacity of these information recording mediums is increasing year by year.
For increasing the recording density of the information recording medium, it is effective to increase the numerical aperture NA of the objective lens or to shorten the light emission wavelength of the light source. For example, in an optical head for a Compact Disc (CD), which is a digital optical disc or recording mainly music signals, the numerical aperture NA of the objective lens and the light emission wavelength of the light source are 0.45 and 780 nm, respectively, whereas, in a so-called optical head for the Digital Versatile Disc (DVD), numerical aperture NA of the objective lens and the light emission wavelength of the light source are 0.6 and 650 nm, respectively. With this DVD, the recording density is improved over that with the CD to render it possible to record picture signals.
The objective lens, used for recording and/or reproducing the information for the routine CD or DVD, is prevalently a single non-spherical lens molded from glass or plastics. This single non-spherical lens, carried by a lens holder, makes up an objective lens device, and is loaded on the optical head. For example, the single non-spherical lens is bonded to and mounted on a reference surface of the lens holder. The lens holder, carrying this single non-spherical lens, is loaded with pre-set accuracy on the optical head.
Recently, a higher recording density and a larger recording capacity of the information recording medium are desired, such that a larger numerical aperture NA of the objective lens and the wavelength of the light radiated from picture signals shorter than e.g., 650 nm are required.
It is however impossible to produce the single non-spherical lens with the numerical aperture NA not smaller than 0.75 because of difficulties in metal die machining and in controlling the eccentricity at the time of lens molding. That is, in machining a metal die for molding the single non-spherical lens with the numerical aperture NA not less than 0.75 the tilt angle of the lens surface with respect to the optical axis in the vicinity of the lens is less than 40xc2x0, so that machining becomes difficult in consideration of the size of the distal end of the cutting edge, such as a diamond byte. Also, if the curvature of the lens surface is increased, the sag (depth along the optical axis from the apex of the lens surface to the outer rim of the lens) is increased to render metal die machining difficult. For this reason, it is retained to be difficult to constitute an objective lens with a numerical aperture NA not less than 0.75 as a single lens.
Recently, as a technique of realizing an objective lens with the numerical aperture NA of not less than 0.75, a double set objective lens has come to be used. With this double set objective lens, the objective lens is constituted by plural lenses to diminish the refractive power of each lens. This renders it possible to increase the radius of curvature of the non-spherical lens surface to manufacture an objective lens with the numerical aperture NA not less than 0.75.
However, with the double set objective lens, with a large numerical aperture NA, an extremely high precision is required as the relative lens position accuracy when assembling the plural lenses into one set. For example, micron order accuracy is required of the eccentricity and spacing between the lenses, whilst the minute order accuracy is required of the lens tilt. By making three-dimensional position adjustment, it is sufficiently possible to assemble the lens to meet this precision requirement. However, this three-dimensional position adjustment is in need of an expensive jig and an advanced position adjustment technique and hence does not lend itself to a mass production process.
As means for positioning the lenses to assemble them to a sole lens unit, it may be envisaged to assemble the respective lenses in the lens holder whose reference portion having a reference surface for mounting the lenses thereon has been formed to high precision.
The reference portion formed in the lens holder is set to a shape capable of positioning the respective lenses as to the lens offset, tilt and the lens-to-lens separation. The lens offset, tilt and separation are referred to below as three elements. By forming the reference portion to high precision with respect to these three elements, the objective lens can be assembled to high precision without requiring the position adjustment of the respective lenses. That is, the objective lens can be assembled to high precision solely by sufficiently optimizing the shape designing of the lens holder.
The shape of the lens holder is hereinafter explained inclusive of the presumed manufacturing method. FIGS. 15 and 16 show the structure of an objective lens device 201 having a first lens 202 and a second lens 203 of the double set objective lens set assembled into a lens holder 204.
The first lens 202 is a lens into which falls the laser light radiated from a light source, not shown. The first lens 202 has its mid portion facing the second lens 203, referred to below as the radiating surface, is formed as a non-spherical lens surface 202a, on an outer rim of which is formed a planar portion perpendicular to the optical axis. On the other hand, the surface to which falls the light radiated from the light source, and which is the opposite surface of the first lens 202, has its mid portion formed to a non-spherical lens surface 202b. On the outer rim of the lens surface 202b is formed a planar portion perpendicular to the optical axis. The above-mentioned opposite surface is referred to below as the incident surface.
The second lens 203 is a lens of the double set objective lens set which opposes to a digital optical disc, such as a phase change optical disc or a magneto-optical disc. The surface of the second lens 203 facing the disc, not shown, referred to below as the facing surface, is formed to a planar shape, whilst the mid portion of the surface facing the first lens 202 as the opposite surface, referred to below as the incident surface, is formed as a non-spherical lens surface 203b. On the outer rim of the lens surface 203b is formed a planar section perpendicular to the optical axis.
The side of the objective lens device 201 on which falls the light radiated from the light source is termed an object point side, whilst the side of the objective lens device 201 lying along the disc arraying direction, that is the side on which an image point is formed by the objective lens device 201 by the light radiated from the light source, is termed an image point side. Thus, with the first and second lenses 202, 203, non-spherical lens surfaces are formed on the object side.
The lens holder 204 is formed substantially to a toroidal shape. On the inner peripheral side of the object point is formed a first mounting portion 204a carrying the first lens 202, whereas, on the inner periphery on the image point side, there is formed a second mounting portion 204b carrying the second lens 203.
The first mounting portion 204a is formed step-wise as one with the inner rim of the image point side aperture, and is formed as one from a first axial reference surface 204a1 formed facing the object point side and from a cylindrically-shaped radial reference surface 204a2 having the optical axis as axis.
The second mounting portion 204b is formed as-one step-wise on the inner rim of the aperture on the image point side from a second axial reference surface 204b1 formed facing the object point side and from a cylindrically-shaped radial reference surface 204b2 having the optical axis as axis. This lens holder 204 is prepared e.g., by molding from synthetic resin.
In the above-described lens holder 204, the first axial reference surface 204a1 of the first mounting portion 204a and the second axial reference surface 204b1 of the second mounting portion 204b serve as a reference surface determining the separation along the optical axis of the first and second lenses 202, 203. On the other hand, the first axial reference surface 204a1 and the second axial reference surface 204b1 also act as a reference surface for controlling the tilt of the first and second lenses 202, 203. It may be said that the orientation of the first axial reference surface 204a1 and that of the second axial reference surface 204b1 are at 180xc2x0 along the optical axis. The radial reference surface 204a2 of the first mounting portion 204a and the radial reference surface 204b2 of the second mounting portion 204b act as a reference surface determining the positions of the first and second lenses 202, 203 along the radius of the lens.
On this lens holder 204, the first lens 202 is mounted by its outer rim 202c on the first mounting portion 204a. The second lens 203 is mounted by its outer rim203c on the second mounting portion 204b. 
This lens holder 204 is produced by die molding. For example, it is manufactured using a male die shown in FIG. 17, referred to below as a first metal die, and a female mold 302, referred to below as a second metal die. When the dies 301, 302 are assembled as shown in FIG. 17, a molding material 204a for producing the lens holder 204 is charged into a cavity between the first metal die 301 and the second metal die 302 to mold the lens holder 204.
In this metal die for manufacturing the lens holder, the first metal die 301 is provided with a molding portion 301a for forming the first mounting portion 204a, referred to below as the first mounting portion molding portion 301a, whilst the second metal die 302 is provided with a molding portion 302a for forming the second mounting portion 204b, referred to below as the second mounting portion molding portion 302a. 
The first metal die 301 is made up of a base 301b and a projection 301c provided on this base 301b, and is generally formed to substantially a convexed shape, as shown in FIGS. 17 and 18.
The base 301b is formed substantially to a flat plate shape. This base 301b is provided with a projection 301c at a mid portion of the major surface 301b1 in a direction towards the second metal die 302. This direction towards the second metal die 302 is the direction towards the image point of the double objective lens set 201 and is referred to below as the image point side direction. The vicinity of the outer rim of the major surface 301b1 of the base 301b is an abutment surface 301b2 compressing against an abutment surface 302b21 of the second metal die 302.
The projection 301c is formed with steps 301d, 301e, 301f which become progressively lesser in diameter in a direction towards an image point side, as also shown in FIG. 19. In this projection 301c, the major surface 301f1 of the step 301f as the distal end of the projection 301c serves as the surface of the second metal die 302 compressing against the major surface 302c1 of the second metal die 302. In this first metal die 301, the first molding portion 301a is made up of a surface 301a1 facing the image point side of the outer rim of the step 301d and an outer rim surface 301a2 of the step 301d. 
The second metal die 302 is made up of a base 302b and a projection 302c set upright at a mid portion on the major surface 302b11. The base 302b is made up of a substantially flat-plate shaped bottom 302b1 and a sidewall section 302b2 set upright on the outer rim of the major surface 302b in a direction of the first metal die 301, that is in a direction towards the object point side of the double objective lens set 201, referred to below as an object point side direction.
In the second metal die 302, the sidewall section 302b2 is a portion lying on the outer rim of the projection 301c of the first metal die 301 and has an end face as an abutment surface 302b21 compressing against the abutment surface 301b2 of the first metal die 301.
The major surface 302c1 of the projection 302c is an abutment surface against the major surface 301f1 of the first metal die 301.
The metal dies 301, 302, thus configured, are assembled as shown in FIG. 17, and the molding material 204a is charged as shown in FIG. 18 to mold the lens holder 204 by die molding.
With the metal dies 301, 302, thus assembled together, the molding material 204a is charged to produce the lens holder 204 whereby the lens holder 204 is formed as one with reference surfaces for positioning the lenses 202, 203, so that these lenses may be assembled to a double objective lens set without increasing cost or time.
In assembling plural lenses, precision in the above-mentioned three elements, that is the offset, tilt and the separation, is required. That is, the lens holder 204 needs to be shaped to this precision.
However, in the above-described lens holder manufacturing method, the first mounting portion 204a or positioning the first lens 202 is formed by the first metal die 301, whilst the second mounting portion 204b for positioning the second lens 203 is formed by the second metal die 302, so that, if the molding material 204a is charged in a state in which the second metal die 302 is not properly positioned with respect to the first metal die 301, there is produced offset in the above-mentioned three elements.
For example, if the molding material 204a is charged in a state in which the second metal die 302 is positioned with an offset from the pre-set position, as shown in FIG. 20, there is directly produced an offset as to the offset, tilt or the separation between the first lens 202 and the second lens 203 assembled in the molded lens holder 204.
Specifically, should the second metal die 302 be offset relative to the first metal die 301, as shown in FIG. 20, there is produced an offset in eccentricity X due to the offset of a center axis O2 of the second metal die 302 with respect to the center axis O1 of the first metal die 301. On the other hand, there are produced an offset xcex8 of tilt due to the inclination of the center axis O2 of the second metal die 302 with respect to the center axis O1 of the first metal die 301 and an offset Z due to separation of the second metal die 302 from the first metal die 301. These offsets represent an amount of deviation between the first molding portion 301a on the first metal die 301 for molding the first mounting portion 204a and the second molding portion 302a on the second metal die 302 for molding the second mounting portion 204b. If the first and second lenses 202, 203 are assembled in the lens holder 204 molded in such a state that the second metal die 302 has such the offsets with respect to the first metal die 301 as to the three elements, as shown in FIG. 21, the offset X in eccentricity, offset Y in tilt and the offset Z in the separation add to the error ascribable to the shape of the metal dies 301, 302, that is to the metal die producing error.
The metal die producing error of approximately 3 xcexcm for the eccentricity and separation and that of approximately 0.02xc2x0 for the tilt may be estimated to be produced, so that, if the first metal die 301 is offset from the second metal die 302, there is produced an error between the first and second lenses 202, 203 in an amount corresponding to the metal die producing error plus the amounts of deviation between the metal dies 301, 302. Since the eccentricity and the separation between the metal dies are estimated to be approximately 10 xcexcm, whilst the tilt of approximately 0.0670 is estimated in die molding, these offsets add up to each other so that the eccentricity and the separation amount to approximately 13 xcexcm, whilst the tilt is approximately 0.087xc2x0.
If, in the above-described lens holder manufacturing method, in which the first mounting portion 204a carrying the first lens 202 and the second mounting portion 204b carrying the second lens 203 are produced using separate metal dies, an offset is produced at the time of assembling the first and second metal dies 301, 302, this offset translates itself as the offset in the reference surface usable as a reference in mounting the first and second lenses 202, 203. The result is that the relative position between the first and second lenses 202, 203 assembled in position deviate from the desired relative position.
It is therefore an object of the present invention to provide a lens holder, a method for manufacturing lens holder, metal die for producing lens holder and objective lens device constructed using the lens holder, in which plural lenses can be secured to high precision without adjustment, and in which relative lens positions can be positioned to high accuracy.
In one aspect, the present invention provides a lens holder carrying an objective lens formed by a plurality of lenses arrayed on an optical axis of an optical head adapted for recording and/or reproducing information signals for an information recording medium using the laser light, with the plural lenses making up an objective lens device. A plurality of reference surfaces respectively carry the object point side lenses for controlling the positions along the optical axes and the tilt of the lenses. The object point side reference surfaces control the positions along the optical axes and the tilt of the lenses.
In this lens holder, the reference surfaces are formed by machining from the same direction.
In another aspect, the present invention provides a method for manufacturing a lens holder carrying an objective lens formed by a plurality of lenses arrayed on an optical axis of an optical head adapted for recording and/or reproducing information signals for an information recording medium using the laser light, by injection molding means, using a metal die including at least a female die and a male die, object point side lenses making up an objective lens device. A reference portion forming portion constituting a plurality of reference portions for object point side lens holder respectively carrying object point side plural lenses are formed on one of object point side male die or the female die.
In this manufacturing method for the lens holder, the reference portions of the lens holder are formed by the reference portion molding portion provided on the male die or the female die.
In still another aspect, the present invention provides a metal die for manufacturing a lens holder having at least a female die and a male die, with the object point side metal die being used for manufacturing a lens holder carrying an objective lens formed by a plurality of lenses arrayed on an optical axis of an optical head adapted for recording and/or reproducing information signals for an information recording medium using the laser light, with the object point side lenses making up an objective lens device. A reference portion molding portion for forming a plurality of reference portions of the lens holder for respectively mounting object point side lenses is formed on one of the female die and the male die.
In the metal die for manufacturing a lens holder, the reference portions of the lens holder are formed by the reference portion molding portion provided on the male die or the female die.
In still another aspect, the present invention provides an objective lens device for an optical head adapted for recording and/or reproducing information signals for an information recording medium using the laser light, with the object point side lenses making up an objective lens device. The object point side objective lens device includes an objective lens made up of a first lens and a second lens, and a lens holder supporting object point side objective lens. At least one surface of the first and second lenses having a mid portion formed as a lens surface, with the vicinity of the outer rim of object point side at least one surface proving a planar section perpendicular to the optical axis. The object point side lens holder has a first reference surface and a second reference surface for mounting object point side first and second lenses, respectively, for controlling the tilt and the positions along the optical axes of the lenses. The object point side reference surfaces are oriented in one direction along the optical axis. A planar portion of object point side first lens is co-planar as the first reference surface to support object point side first lens, with a planar portion of object point side second lens being co-planar as the second reference surface to support object point side second lens.
With the lens holder of the present invention, in which there are formed plural reference surfaces for mounting plural lenses for controlling the tilt and the positions along the optical axis direction of the respective lenses are provided in one direction along the optical axis direction, the reference surfaces can be formed by machining from the same direction.
By this lens holder, plural lenses can be positioned and secured to high precision in an adjustment-free fashion to enable high-precision positioning between the respective lenses.
In the lens holder manufacturing method according to the present invention, in which there are used metal dies carrying a reference portion molding portion adapted for forming plural reference portions of the lens holder, in one of the male and female dies of which the plural lenses are mounted, the reference portions of the lens holder can be formed by the reference portion molding portion formed on one of the male and female dies.
With the lens holder manufacturing method, plural lenses can be secured to high accuracy in an adjustment-free fashion, so that positioning between the lenses can be realized to high precision.
In the metal die for manufacturing the lens holder according to the present invention, in which there is provided a reference portion molding portion for forming plural reference portions of the lens holder, on a male die or a female die of which plural lenses are mounted, the reference portions of the lens holder can be formed by the reference portion molding portion formed in the male die or in the female die.
With the metal die for manufacturing the lens holder, it is possible to manufacture a lens holder on which plural lenses can be positioned to high precision in an adjustment-free fashion, such that the positioning between the lenses can be achieved to high precision.
On the objective lens device of the present invention, having the above-mentioned lens holder, plural lenses of the objective lens constituting the objective lens are secured in position to high precision in an adjustment-free fashion.